Higher Quality Images for a Visible-Light CMOS Sensor by Suppressing Spatial Row-Wise Noise with an Output-Capacitor-Less, Internal Low-Dropout Regulator

Image quality of visible-light CMOS sensors normally suffers from fixed-pattern noise. Spatial row-wise noise in an image particularly is caused by pixel array power supply coupling. Common noise injection from a fluctuating power supply during pixel readout causes horizontal stripe noise because all signals in a row are affected by the noise simultaneously. By integrating an output capacitor-less low-dropout (LDO) regulator on chip to directly supply a regulated power to the imager pixel array exclusively, it is possible to reduce fixed-pattern row-wise noise considerably so that the row-stripe noise becomes invisible entirely. The LDO circuit uses internal inherent pixel array parasitic capacitances along with an integrated capacitor multiplier circuit to eliminate the need for an off-chip capacitor and thus isolates the pixel array from the external board components such as bond-wire, pad, and board traces. The LDO was designed and fabricated in 0.13 mu m - 2P4M CMOS image sensor process and was implemented within a five-megapixel visible-light CMOS imager to prove the functionality and performance. Experimental data are included and shown that the row-wise noise is reduced by more than factor of five, making it negligible (and invisible) compare to the temporal pixel noise.